Containers for preserving hydrocarbons, various chemicals, bathroom furnishings, sanitary products, cosmetics, beverages, and food pastes includes, for example, a fuel tank for cars or small engines, a bottle, a can, and a tube. In many cases, metal and glass conventionally used as the material are substituted with plastic. Plastic is characterized by saving the weight, eliminating rust prevention treatment, reducing breakability, and improving the degree of freedom of shape.
Many of the containers for preserving various chemicals, bathroom furnishings, sanitary products, cosmetics, beverages, and food pastes are formed by polyolefins such as high density polyethylenes (hereinafter sometimes abbreviated as “HDPE”), linear low density polyethylenes (hereafter sometimes abbreviated as “LLDPE”), polypropylenes (hereafter sometimes abbreviated as “PP”), and polyesters such as polyethylene terephthalates (hereafter sometimes abbreviated as “PET”). Many of the containers have excellent mechanical strength, formability, design, and economic efficiency. However, the containers have the disadvantages that the component of the stored object disperses in the atmosphere through the wall of the containers to impair the function of the stored object and that oxygen enters from the outside through the wall of a container to oxidize the stored object so as to impair the taste.
To eliminate these disadvantages, the technology imparting a gas barrier property to the plastic container is used. For example, the method of forming a multilayer structure by layering a barrier resin such as an ethylene-vinyl alcohol copolymer resin (hereinafter sometimes abbreviated as “EVOH”) as the interlayer of a plastic container is known (see Patent documents 1 and 2). The method of manufacturing a single-layer container from the composition in which a polyamide such as nylon 6 or 6/66 and HDPE are blended with an adhesive resin is also known (see Patent documents 3 and 4). Furthermore, the method of manufacturing the single-layer container by using polymetaxylylene adipamide (hereafter sometimes abbreviated as “N-MXD6”) with a more excellent barrier property than that of a polyamide such as nylon 6 is disclosed (see Patent documents 5 and 6).
Conventionally, such a container is manufactured in manufacturing facilities equipped with an extruder 100 and a cylindrical die 110 as shown in FIGS. 5 and 7. The above-mentioned resin is fed to the extruder 100, melted and mixed, formed in a cylindrical shape, passing through the cylindrical die 110, and extruded as a cylindrical parison from an outlet 114 in the lower part 112 of the cylindrical die 110.
Generally, the cylindrical die 110 producing a single-layer container is provided with a die body 120 having a hollow 122, and a mandrel 130 placed in the hollow 122 of the die body 120, the mandrel 130 forming resin flow paths 150.
As shown in FIGS. 5 and 6, the mandrel 130 has a recess 132 with a shape of a heart or a spiral, which is fixed to the upper part 116 of the cylindrical die 110 so as to form the resin flow paths 150 in the hollow 122. In the cylindrical die 110 equipped with this mandrels 130, melted resin fed from the extruder 100 to a resin inlet provided in the mandrel 130 is divided by the cylindrical side of the mandrel 130 to flow into two directions. The divided resin flows around the mandrel 130 along the resin flow paths 150 and the recess 132 formed on the mandrel 130 to be gradually formed in a cylindrical shape, and then extruded from the outlet 114 of the cylindrical die 110 as a cylindrical molding (parison).
As shown in FIG. 7, in the cylindrical die 110, the part where the melted resin joins together to be formed in a cylindrical shape is generally referred to as “weld”. For example, when a cylindrical die 110 provided with a heart-shaped mandrel 130 is used, melted resin fed from the extruder 100 to the cylindrical die 110 flows downward from the inlet provided on the mandrel 130 for the resin flow paths 150 and is divided by the cylindrical side of the mandrel 130 to flow in the left and right directions. Since the right and left flow paths are shallower toward their ends, the melted resin gradually overflows from the flow paths and flows obliquely downward. Finally, the melted resin joins together at the opposite side to the part where the melted resin is divided. This part where the melted resin joins together is the weld 160. Even in a cylindrical die 110 provided with a double-heart shaped or spiral-shaped mandrel 130, melted resin fed from an extruder flows from the side of the mandrel to resin flow paths provided on the mandrel 130, in the same way. The tip of the resin flow joins at a part along the mandrel 130, which forms a weld 160.